In casting of a steel, a nozzle for casting which is a flow path of a melted steel discharged from a smelting vessel including a ladle and a tundish, and an SN device which controls a flow rate of a melted steel are used. In this SN device, two or three SN plates having nozzle holes which are made of a refractory are used. These SN plates are piled up under a restricted condition and moved under a state applied with a surface pressure, wherein the flow rate of the melted steel is controlled by adjusting an opening degree of the nozzle hole.
Because of this, the SN plate is required to have characteristics including the mechanical strength endurable to a use under the restricted condition, the thermal shock resistance to a thermal stress during the time of casting, the corrosion resistance and the oxidation resistance to the components present in a melted steel, a slag, and the like, and “the surface-roughing resistance”, a resistance to “the surface roughness”, i.e., an attrition received in a moving surface which is an operation surface.
In the SN plate, an alumina carbonaceous refractory is generally used; and the refractory is roughly classified, depending on the production method thereof, into a high-temperature baked product which is baked at a high temperature of about 1000° C. or higher and a low-temperature baked product which is baked at a temperature of lower than about 1000° C. Generally, the high-temperature baked product has a higher porosity than the low-temperature baked product due to a change in the organization thereof during baking; and thus, the former is produced by further impregnation with a tar, a pitch, or the like. Therefore, the high-temperature baked product has the organization having high density and strength because of the baking at a high temperature as well as the impregnation; and in addition, because this product is baked at a high temperature, not only it is thermally stable to a high temperature region but also it has an excellent characteristic in the thermal shock resistance. However, this product undergoes the processes including baking at a high temperature, impregnation with a tar or a pitch, and coking after the impregnation so as to previously remove a harmful substance and a substance which generates a smoke during the use thereof, so that a cost of production of the SN plate becomes very high in terms of an energy and a process; and in addition, an environmental care becomes necessary because this method uses a tar, a pitch, or the like.
On the contrary, the low-temperature baked product has merits that the energy cost can be made lower and the process is environmentally more friendly as compared with the high-temperature baked product. In order to furnish this low-temperature baked product with the strength and the oxidation resistance, a metal aluminum or a metal aluminum-containing alloy, each having a low melting point, is added thereto; and many products, so-called light baked products or so-called non-baked products, which are baked in various temperature regions below about 1000° C., have been disclosed.
For example, Patent Document 1 discloses the production method in which after a blend of a refractory raw material, a phenol resin, and a metal aluminum powder including an atomized powder in the ball-like form is kneaded and molded, this blend is subjected to a heat treatment in the temperature range of 550 to 650° C. It is reported that when the temperature of the heat treatment is below 550° C., not only the oxidation resistance of the phenol resin decreases but also a decomposition gas is generated thereby generating an odor during the use thereof, while when the temperature is above 650° C., an aluminum carbide is formed. It is reported that when the aluminum carbide is formed, this reacts readily with water at a normal temperature and a normal pressure to form a metal aluminum hydroxide with accompanying a volume expansion and a weight increase, thereby often causing collapse of the plate during the storage thereof.
Patent Document 2 discloses the method in which a phenol resin is added to a blend containing 90 to 99.5% by mass of an aggregate of a fire resistant inorganic material and 0.5 to 10% by mass of a metal aluminum fiber or a metal aluminum alloy fiber, and then, the resulting mixture is subjected to a heat treatment at 700° C., 850° C., or 1000° C. It is reported that by carrying out the heat treatment at a temperature higher than a melting point of the metal aluminum or of the metal aluminum alloy (melting point of the metal aluminum is 660° C.), the metal aluminum penetrates into among the particles in the neighborhood organization thereof, thereby not only the strength of the refractory can be dramatically improved but also the thermal shock resistance can be significantly enhanced. It is also reported that when the temperature of the heat treatment is higher than 1000° C., excellent characteristics of the metal aluminum or of the metal aluminum alloy as the fiber cannot persist, thereby not only leading to an indifference in the characteristics between the fiber and the powder but also forming, as penetration of the metal aluminum progresses, a void in the place where the fiber was present, which can rather deteriorate the corrosion resistance.
Patent Document 3 discloses the production method in which a phenol resin is added to a raw material including a fire resistant inorganic raw material, a carbonaceous raw material, and a metallic raw material, these raw materials constituting a continuous particle size distribution system with the particle diameter thereof being in the range of 0.1 to 4000 μm, both inclusive, and then, the resulting mixture is baked in a non-oxidative atmosphere in the temperature range of 800 to 1500° C. without carrying out the impregnation treatment. In Example therein, the refractory which is baked at 850° C. and whose apparent porosity is 5.0% is disclosed.
Patent Document 4 discloses the refractory which is produced by adding an organic binder to a refractory raw material blend including 0.5 to 20% by mass of a metal aluminum and/or a metal aluminum alloy followed by subjecting the resulting mixture to kneading, molding, and a heat treatment in the temperature range of 400 to 1000° C., both inclusive, but not followed thereafter by impregnation with a carbonaceous liquid substance including a tar and a pitch, thereby affording the refractory with the compressive strength of not less than 180 MPa and the weight increase rate of not more than 1% by a slaking test with an autoclave.
As disclosed in Patent Documents 1 to 4, in general, conventional refractories for casting, especially a refractory for an SN plate (hereinafter, referred to as “plate refractory), which is required to have excellent properties including the corrosion resistance and the abrasion resistance, is composed of mainly oxide materials including an alumina-based oxide, a magnesia-based oxide, a spinel-based oxide, and a zirconia-based oxide. However, these oxides have a problem of an insufficient thermal shock resistance because of a large thermal expansion rate.
In the refractory for casting, especially in the plate refractory, with regard to the method for enhancing the thermal shock resistance thereof, a method using a raw material containing a silica including a mullite, or a method concurrently using with it a raw material containing a zirconia including a zirconia mullite and an alumina zirconia is often employed.
A raw material containing a silica (silica-containing raw material), for example, a mullite, has a lower thermal expansion rate in the mineral itself than a mineral constituting an alumina-based oxide, a magnesia-based oxide, a spinel-based oxide, and a zirconia-based oxide, or the like; and therefore, by decreasing the thermal expansion rate as a refractory by adjusting the contents thereof, the thermal shock resistance of the refractory can be improved.
A raw material containing a zirconia (zirconia-containing raw material) has a lower thermal expansion rate as compared with an alumina raw material; and in addition, because of a specific expansion behavior accompanied with a zirconia-specific phase transition, a microcrack or a microspace is formed in the organization thereby generating an effect to decrease in the modulus of the refractory; and therefore, it is presumed that the thermal shock resistance is afforded by the effects of the decreases in the thermal expansion rate and in the modulus.
In order to improve the thermal shock resistance by concurrently using these raw materials containing a silica or a zirconia, it is necessary to have a comparatively large amount of them contained therein, for example, in the range of about 5.0 to 15.0% by mass. However, use of such a large amount of these raw materials can rather cause a decrease in the tolerance thereof.
Namely, the silica component is readily reduced in an atmosphere co-existing with a carbon to become an SiO gas which can easily disappear to make the refractory organization less dense, so that an iron-based oxide or a slag component can readily infiltrate deep into the organization; and in addition, the oxidation resistance is prone to be decreased. Further, the silica component reacts with an iron-based oxide derived from a melted steel, a steel inclusion, and a slag component to form low-melting point substances thereby leading to a dissolution loss. Accordingly, when a large amount of the silica component including a mullite and a zirconia mullite is contained therein, the tolerance thereof decreases due to the decreases in the corrosion resistance, the oxidation resistance, and the like.
In the case of the zirconia component, because an effect including an effect of a stress relaxation due to the microcrack, the microspace, or the like is utilized; this is effective in the case that the refractory is used less repeatedly, or in a comparatively mild use condition including a comparatively short period of a casting time, or the like. However, in the refractory concurrently using the zirconia-containing raw material, under the condition of the use for a long period of time or of the repeat use for many times, a damage or the like due to an edge defect, which is caused by expansion of the crack or deterioration of the organization, an abrasion of the moving surface, or the like, increases, thereby rather causing a decrease in the tolerance thereof.
Meanwhile, the repeat use herein means as follows. That is, in the case of using thereof as the SN plate for a ladle, or even as the tundish used under the condition of a hot rotation or the like, after casting under the high temperature condition in which the temperature around a nozzle hole is not lower than 1000° C., the plate itself is cooled to a temperature condition of not higher than about 500° C. until next casting, namely, meaning the repeat use condition between heating at a high temperature and cooling. The multiple repeat use means the use condition of plural ch (for example 8 ch or more) in the case of the ladle, and the use condition of 2 or more castings in the case of the hot rotation tundish.
Also, the use for a long period of time means the long time condition during receiving of a steel with a total time of casting not less than about 500 minutes in the use as the ladle, and the condition of not less than 800 minutes in the use as the hot rotation tundish.
These use conditions cause a change in the organization of the plate refractory because of the repeated heating and cooling as well as the exposure to the high temperature conditions for a long period of time. Therefore, these are severe use conditions to cause an increase in an attrition of the plate refractory.
Many of the refractory for casting employ a carbon bond in the bonding organization thereof. Therefore, in order to protect this carbon from being lost by oxidation, a metal represented by a metal aluminum which has a high oxygen affinity is often used concurrently. The metal aluminum is also applied to the refractories baked at a low temperature or at a high temperature as disclosed in Patent Documents 1 to 4.
On the other hand, when a zirconia-containing raw material is concurrently used with a refractory which is applied with and contains a low-melting point metal including the metal aluminum which has a high oxygen affinity as mentioned above, this becomes one cause to decrease the tolerance of the refractory.
The cause and mechanism thereof may be presumed as follows.
Under the condition in which an area around the nozzle hole of the nozzle for casting, a moving surface of the SN plate, and the like are exposed to a high temperature, the atmosphere inside the pore of the refractory becomes a reductive atmosphere because a carbon is present therein, too. In addition, when the metal aluminum is present in the organization, an oxygen concentration therein further decreases, resulting in a highly reductive atmosphere. In this highly reductive atmosphere, not only a silica but also a zirconia is readily reduced, thereby reacting thereafter with the carbon to form zirconium carbide, zirconium carbon monoxide, and zirconium. The zirconium carbide, zirconium carbon monoxide, and zirconium thus formed have a high oxygen affinity, so that they are readily oxidized under an oxidative atmosphere to form a zirconia. Upon this, the volume expansion takes place to generate a defect in the organization. As a consequence, during the time of casting for a long period of time or during the repeat use, the refractory organization is deteriorated to cause various damages as described before.
The substance like the metal aluminum, which has a high oxygen affinity as mentioned before, is highly effective as an antioxidant; but when the substance is exposed to the high temperature condition for a long period of time in the refractory organization, the oxide raw material in the refractory is reduced to cause a change in the properties thereof, thereby also causing an adverse effect such as deterioration of the organization as the refractory or a decrease in the tolerance thereof.
Meanwhile, other metals including magnesium generate the reactions similar to that of the metal aluminum because of the oxygen affinity thereof, though the temperature range of the reactivity and the like are different depending on the metals. Accordingly, an addition of a raw material containing a silica or a zirconia including an alumina zirconia and a zirconia mullite, all of which have been used in conventional technologies and the amount of which is large but about just enough to improve the thermal shock resistance thereof, to a refractory containing the metal aluminum, magnesium, and an alloy containing them, has a limit in improvement of the tolerance in thermally severe conditions including the casting for a long period of time and the multiple repeat use.
The reaction of the metal aluminum with the raw material containing a zirconia or a silica as described above or the degree of the damage in the refractory is different depending on the relative contents of these substances and the forms in their presence. Also, if a large amount of the metal aluminum is used in the refractory composition like this, the refractory is densified with an increase in the modulus by the reaction of the metal aluminum, thereby leading to a decrease in the thermal shock resistance; and thus, the repeat use thereof, especially in the SN plate of a large size, becomes difficult.
On the other hand, Patent Document 5 discloses the SN plate wherein a content of Al4O4C as a mineral phase is in the range of 5 to 95% by mass, the thermal expansion rate thereof is 8×10−6/K or less, and the flexural modulus thereof at a normal temperature is in the range of 10 to 60 MPa, both inclusive.
Because Al4O4C is low in the thermal expansion, it is expected that this may contribute to improvement in the thermal shock resistance. However, when Al4O4C is exposed to the oxidative condition for a long period of time, or subjected to the multiple repeat use, or the like, the oxidation thereof to Al2O3 advances; and therefore, the effect thereof as the low expanding base material decreases. Accordingly, only by including Al4O4C therein, an improving effect in the thermal shock resistance or in the tolerance cannot be obtained sufficiently.